Devices that produce optical TTDs can be used for the steering of radar phased arrays, transversal filtering, and other optical signal processing applications. Electronically implementing TTDs is generally impractical because such implementation requires long lengths of strip lines, waveguides, or coaxial cable, which are expensive, bulky, and temperature sensitive. Because long paths are comparatively easy to obtain optically, photonic systems provide a means of obtaining a combination of the beam agility of array systems and wide bandwidth. Approaches to TTD devices tend to fall into two categories: those using fibers and those using long free-space paths. Some fiber approaches use multiple optical switches or broadcast the light over multiple possible paths at once. In addition, wavelength-division-multiplexing schemes have recently been developed by use of fiber Bragg gratings.
Free-space systems have used multiple optical switches for switching the beams between sequential optical paths. These optical switches are usually liquid-crystal based. Another type of free-space system includes a TTD device that uses a multiple-pass optical cell with refocusing mirrors.